An Inside Look at Sanctum's Terrifying Caves

Producer Andrew Wight used a harrowing real-life experience as inspiration for Sanctum, out February 4. He walks Popular Mechanics through the gonzo engineering that brought the story to the big screen.

In 1988, Andrew Wight was leading an expedition of 14 people in exploring a large underwater cave in Australia's Nullarbor Plains when a storm flooded the entrance and caused the cave to collapse, trapping them inside. "It took nearly two days to find a new way out of the cave," Wight says. "It was one of those moments where you come very close to your own demise and afterward you start reflecting on what has happened. The struggle to survive was the thing that really struck me. People who you thought might become the heroes and the leaders of the group don't necessarily do what you think they're going to do, and others rise to the occasion."

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This real-life event was the inspiration for Sanctum, out February 4. In the film, diver Frank McGuire (Richard Roxburgh) is trying to map a previously unexplored cave system; he's enlisted his 17-year-old son, expert climber and reluctant volunteer Josh (Rhys Wakefield), to help. Billionaire Carl Hurley (Ioan Gruffudd), who is funding the expedition, brings along his girlfriend Victoria (Alice Parkinson) to check out the caves. Then a flash flood cuts off the entrance and everything goes to hell.

Wight initially wanted to film Sanctum in real caves, so filmmakers searched all over the world for the perfect spot. But it quickly became clear that filming in real locations would be too dangerous. So Wight came up with another idea. "With my experience in cave exploration, being to lots of places in the world, I said 'You know what? We can create the environments that we need to film, and then we'll do second-unit stuff in the real caves,'" he says.

Designing the faux caverns was no easy task: The sets needed to accommodate all the needs of the production, including camera cranes. "We didn't want have to continually reconfigure things," Wight says. "From the ground up, everything was carefully planned. When you're on a tight budget and a tight timeline, you can't afford to slow down because you've got to move the camera around."

Set designers used photographs from real caves to create a CAD model of the set, then built a scale model, in clay, of what they intended to build at the Warner Bros. studios on the Gold Coast of Australia. Finally, it was time to construct the actual set pieces and put them together in the facility's huge tank. "The sets were constructed out of concrete, and sculpted to faithfully replicate the cave environment," Wight says. "It was not a friendly set. There was flowing water, boulders, rocks. If you fell over, you were going to hurt yourself. If you didn't know you were inside a studio, you would really think you were in a cave." Two sump pumps flooded the tank and submerged the entire set, which was built modularly so pieces could be reused to create different cave environments. (The pieces were later recycled into a putt-putt golf course.)

Sanctum was shot in 3D with the Fusion Camera system developed by James Cameron and Vincent Pace. Filmmakers used four stereoscopic cameras: Studio shots were captured with two split-beam rigs created for Avatar, while two side-by-side rigs were used for helicopter shots and underwater photography in the caves of Australia's Mount Gambier. (Multiple cameras, however, were used to get various angles of stunts.) Getting the 3D cameras into real caves was tough. "The entrances were pretty small," Wight says. "We had to hand-lower it, and haul everything out. That was pretty challenging, but we've done it all before. It's not the first barbecue for me."

The biggest technical challenge, Wight says, was working with water. "You've got a million-dollar electronic camera, and you have a million liters of water," he says. "Those two things do not go together." Completely submerging the cameras was simple enough; filmmakers placed them in a waterproof housing. But filming in the studio, where splash and humidity could put water on the lens, was the real issue. "The problem with the 3D photography is that if you get a drop on one lens, the shot becomes very difficult for anyone to look at," Wight says. "You'd have to digitally remove that imperfection in one eye."

To keep water off the lenses, key grip Adam Kuiper looked into air-based blowing systems that were developed for cameras. But Sanctum's rigs were much bigger, and there was a lot more water involved—and there was no time to build anything. "So Adam went, 'Well, who else uses air?'" Wight says. "He stumbled across this air-knife system used in the dried-fruit industry." Kuiper was able to modify the off-the-shelf equipment; he affixed the device to the front of the camera, then attached it to a long hose that led to a compressor, operating at 190 psi, outside the set. The high-pressure curtain of air successfully kept droplets off the lens. Of course, there were other problems: "horrendous sound issues," Wight says. "It kept the water off, but we couldn't use any of the sound. But we have the technology to replace the sound in post, so that wasn't as big an issue as keeping the cameras dry."

It might seem like a lot of trouble to go through, but Wight believes 3D provides a better movie-watching experience. "The final part of that journey of creating cinema, which replicates how we see the world, is that you use stereo," he says. "If 3D is done well, it shouldn't draw attention to itself; it's just part of the story. We're not using 3D as a gimmick. But what it does do, in this kind of movie, is make the audience feel like they're closer to what's going on. They've really got that sense of being there, more so than if you're watching it in 2D."